1. Field of the Invention
The present invention relates to a process for the preparation of curable fluoroelastomers, and to the products so obtained.
Fluoroelastomers, and in particular fluoropolymers on the basis of vinylidene fluoride, hexafluoropropene and other monomers, such as vinyl fluoride, tetrafluoroethylene, and so forth, can be used in several fields thanks to their properties of very good chemical and heat resistance, and to their characteristic of withstanding weathering and of having optimum stability to light.
Such products can be subdivided into two groups, viz. those belonging to high molecular weight polymers, and those belonging to medium-low molecular weight polymers.
The products belonging to the first class are mainly used to prepare poured sheets, to produce coatings adherent to metal surfaces, or in general, for compression moulding.
The products belonging to the second class are very suitable to be used in thermoforming processes, such as the injection moulding or extrusion processes.
It is clear that the molecular weight distribution depends on the type of polymerization process, and on the corresponding reaction conditions; for example, the polymerization of such monomers as those hereinabove mentioned, in aqueous emulsion, in the presence of water-soluble initiators, and at temperature comprised within the range of from 40.degree. to 50.degree. C., yields as a rule a high molecular weight fluoropolymer, having a molecular weight distribution unsuitable for use in thermoforming processes.
2. Prior Art
Various methods have been proposed for reducing the molecular weights of fluoropolymers polymerized in aqueous emulsion, among these, the method consisting in using higher reaction temperatures, or an amount of free-radical initiator greater than the strictly necessary.
Such systems have been proved to be not much valid, both because regulating the distribution of molecular weights is difficult, and because side effects occur, which impair the heat stability of the end product.
Due to such reasons, the only valid method to be able to regulate and keep at medium-low values the molecular weight of fluoropolymers has been the use of suitable chain-transfer agents during the reaction step.
Also in this case drawbacks have to be faced, because some of these products tend to inhibit the polymerization reaction, by slackening the reaction kinetics; other products are active in particular processes only, e.g., in high-pressure processes; still others tend to interfere, during the polymerization, with vinylidene fluoride.
Bromine- or iodine-based chain regulators have been finally proposed, which have succeded in allowing the molecular weights to be regulated without causing such drawbacks as those described above.
It is however important to realize that, in the particular case of fluoroelastomers, the thermoforming is not the only problem to be faced, because for such products also having high characteristics and chemical-physical properties is important in view of the end uses they are intended for.
Fluoroelastomers indeed, as cured by processes of peroxy type, are known in technical literature because they are widely used in the industrial world thanks to their properties of high heat stability and of chemical resistance, also at high temperature.
Due to these characteristics, they are used in particular applications, above all in contact with organic and inorganic fluids characterized by high chemical aggressivity and under severe temperature conditions.
Application examples can be membranes for pumps for particular liquids, such as the membranes for fuel pumps for internal combustion engines, valve seats, non-return valves, flexible hoses, gaskets, and in particular gaskets of the oil gasket type etc.
In all these cases, fluoroelastomers are esteemed because they associate to their properties of chemical and heat, resistance the possibility of being processed in an easy way, e.g., by extrusion, and with more or less complex shapes and profiles of the end manufavture, whatever it may be.
Besides this, these products, to be able to be used in industrial applications, must also display good mechanical properties, above all when reference is made to the ultimate tensile strength, modulus, ultimate elongation, etc.
In general, therefore, fluoroelastomers have to be materials having characteristics of chemical-thermal resistance, mechanical strength and properties of machinability and/or processability as high as possible.